In the marine environment, construction personal are often faced with task of positioning large, three dimensional structures within confined spaces. For example, during construction in ports and other areas, it is common for structures such as bridge sections and tunnel sections to be moved by crane and barge into a desired location. Also, during the construction of oil-rig platforms, it is common for large pilings to be placed on the ocean floor using a piling-rig.
Accurate, on going determination of the position of structures such bridge sections is required during all phases of the construction operation. In order that the bridge be properly configured at completion, horizontal and vertical displacements of even a few centimeters must be avoided. Displacements which are accidentally built into the path of a bridge, for example, may require expensive correction before the bridge can be placed in operation.
Traditional methods of monitoring the placement of structures such as a bridge under construction involve the use of a variety of surveying techniques. These may include visible lasers and/or optical theodolites. Such methods are not particularly convent because they require trained personal to visit the construction site each time a new bridge section is being placed.
Modem surveying methods are able to make use of remote positioning systems which may be satellite- or land-based. The satellite system most commonly used today is the Global Positioning System (GPS) although others, such as the Global Orbiting Navigation System (GOLNASS) are also in use or under development. Some land-base systems which effectively simulate the satellite-based systems for a small area are also in development. These generally involve the use of pseudolites. Engineering and surveying methods which use these systems can be markedly more efficient and accurate than traditional methods.
Use of GPS in surveying typically involves a single operator who sets up a base station with an antenna positioned over a control point having a known position. The surveyor then moves about a construction site with a mobile unit having its own antenna. The operator places the mobile antenna over unknown survey points in the field to record their position using signals transmitted by a number of in-view satellites. A vector or baseline is determined from the basesite to the mobile unit. Survey points whose positions are measured in this fashion may, in turn, become control points from which measurements of other unknown points may be taken. The use of GPS during a survey procedure may be combined with other techniques such as distance and angle measurements taken with GPS stations which may be placed at one of the control points.
Each GPS satellite transmits two coded L-band carrier signals which enable some compensation for propagation delays though the ionosphere. Each GPS receiver contains an almanac of data describing the satellite orbits and uses ephemeris corrections transmitted by the satellites themselves. Satellite to antenna distances may be deduced from time code or carrier phase differences determined by comparing the received signals with locally generated receiver signals. These distances are then used to determine the antenna position. Only those satellites which are sufficiently above the horizon can contribute to a position measurement, the accuracy of which depends on various factors including the geometrical arrangement of the satellites at the time when the distances are determined.
Distances measured from an antenna to four or more satellites enable the antenna position to be calculated with reference to the global ellipsoid WGS-84. Local northing, easting and elevation coordinates can then be determined by applying appropriate datum transformation and map projection. By using carrier phase differences in any one of several known base or mobile receiver techniques, the mobile antenna coordinates can be determined to an accuracy on the order of ±1 cm. Using such real time kinematic (RTK) techniques, an operator can obtain position measurements within seconds of placing a mobile antenna on a unknown point. In RTK systems, satellite correction data is transmitted by a radio or other link between the base and mobile receivers, whether or not there is a clear line of site to ensure that accuracy in the mobile position measurements is maintained and the survey information is correct.